The present invention relates to a method and an apparatus for manufacturing a pneumatic tire in which a lug groove is formed on a tire tread surface.
On a tire tread surface of a pneumatic tire, there are formed a plurality of lug grooves directed about perpendicularly to circumferential direction of the tire and arranged in a row along the circumference of the tire in order to increase friction to the ground and therefore traction force. Especially, a tire for a large-sized construction vehicle has large lug grooves.
Such lug grooves on the tire tread surface are formed when a green tire is vulcanization-molded in a mold. The mold has lug groove ribs for forming the lug grooves, and the lug groove ribs cut into the tread rubber to form the lug grooves when the green tire is vulcanization-molded.
In case of a tire formed with deep lug grooves such as a tire for construction vehicle, volume of rubber removed by the lug groove rib cutting into the tread rubber is large. Therefore, a pressing force by the lug groove ribs arranged circumferentially at regular intervals acts on a belt member inside of the tread rubber largely, so that such inconveniences that a large waving of the belt member (belt wave) is caused, belt gauge (thickness of the belt member) becomes uneven or a joint portion is broken away, are apt to occur.
Especially, regarding a tire for a construction vehicle which uses a belt of low expansion rate as a tire component member, in order to prevent the lug groove rib from biting at the green tire when the mold is closed and in order to remove the lug groove rib from the produced lug groove smoothly when the mold is opened, a split mold type vulcanization-molding machine having split mold parts which are obtained by splitting the mold in a circumferential direction of the tire and movable in radial directions must be used.
However, the above-mentioned vulcanization-molding machine has a complicated construction and requires a large installation space and a high equipment cost. Especially, in case of the tire for large-sized construction vehicle, much more installation space and equipment cost are required.
The present invention has been accomplished in view of the foregoing and an object of the invention is to provide a method and an apparatus of simple construction for manufacturing a pneumatic tire in which the belt wave and unevenness of the belt gauge are restrained to a minimum, joining of the joint portion can be ensured, the installation space is smallest within the limit of necessity, and the equipment cost is low.
In order to achieve the above object, the present invention provides a method for manufacturing a pneumatic tire in which a green tire having tire components assembled is charged in a mold for vulcanization-molding and lug grooves are formed on a tire surface. The method includes previously forming curved grooves at positions on a surface of the green tire corresponding to the lug grooves extending in substantially the same direction as the lug grooves.
Since carved grooves are formed previously, before the green tire is vulcanization-molded, at portions on a surface of the green tire where lug grooves are to be formed extending in substantially the same direction as the lug grooves, volume of rubber displaced by lug groove ribs of the mold of the vulcanization-molding machine cutting into the tread surface is small and pressing force caused by the lug groove ribs is small, so that unevenness of the belt wave and the belt gauge caused by the pressing force acting on a belt member inside of the tread rubber can be restrained small, and joining of joint portions can be ensured.
In the above-mentioned pneumatic tire manufacturing method, the carved groove may be carved in a shape opened from a tread center side to a tread end. Carving work can be carried out easily.
The carved groove may be carved In a shape opened from a neighborhood of a closed end point of the lug groove to the tread end. The closed end point of the lug groove means a tip end point of the lug groove on the tread center side where the lug groove rib of the mold touches the green tire firstly when the mold is closed. Since the carved groove is carved In the above-mentioned shape, the lug groove rib enters the carved groove smoothly on molding and a predetermined lug groove can be formed accurately.
The closed end point of the lug groove may be distant from a tread center by a distance larger than 0.1 times of a tread width. Since the carved groove directed about perpendicularly to circumferential direction of the tire is carved on a surface of the green tire in a shape opened from the closed end point distant from a tread center by a distance larger than 0.1 times of a tread width to a tread center, the carving work can be carried out more easily.
A main part of the lug groove may be inclined with regard to axis of the tire at an angle of 5 degrees and more and 45 degrees and less. Since the carved groove extending in the same direction as the lug groove is also inclined with regard to axis of the tire at an angle of 5 degrees and more and 45 degrees and less, the lug groove rib of the mold enters the carved groove easily on molding and a predetermined lug groove can be formed accurately.
The carved groove may be carved in a shape opened from one tread end to another tread end. A lug groove extending continuously from one tread end to another tread end can be formed easily and a tire exhibiting excellent traction force can be manufactured.
The carved groove may be carved in a substantially triangular shape opened from a tread center side to a tread end gradually widening. The lug groove rib enters the carved groove easily when the mold is closed and a full mold composed of upper and lower split half parts can be adopted in the vulcanization-molding machine easily. Further, volume of space at a part of the carved groove not overlapping with a lug groove formed by the lug groove rib can be made so small that the space can be filled with a rubber flowing into it easily on vulcanization-molding. Thus, wrinkles caused by the space not filled with the rubber can be restrained effectively to improve appearance of the tire.
The carved groove of the gradually widening shape may be formed by carrying out the carving twice using a cutter. The gradually widening carved groove can be formed easily without necessitating a special cutter.
The carved groove may be carved along direction of the lug groove and along direction of axis of the tire from or toward a neighborhood of the closed end point of the lug groove. The carved groove can be carved along direction of the lug groove from a neighborhood of the closed end point or toward the neighborhood of the closed end point, so that an optimum carving direction can be selected in accordance with condition of the equipment.
Volume of the carved groove may be 0.4-1.2 times of volume of the lug groove. If volume of the carved groove exceeds 0.4 times of volume of the lug groove, it is possible to lessen influence of the vulcanization-molding on the belt layer to restrain the belt wave small and ensure joining of the joint portion. If volume of the carved groove is less than 1.2 times of volume of the lug groove, it is possible to restrain generation of bear and avoid poor vulcanization.
Volume of the carved groove may be 0.7-1.0 times of volume of the lug groove. Generation of the belt wave and the bear can be prevented about surely.
Depth of the carved groove may be 0.5-0.9 times of depth of the lug groove. If depth of the carved groove is less than 0.5 times of depth of the lug groove, volume of rubber displaced by the lug groove rib of the mold on vulcanization-molding is too much, and if depth of the carved groove is larger than 0.9 times of depth of the lug groove, gap between the carved groove and the lug groove rib becomes large, to cause such inconveniences that the bear is liable to be generated to bring about poor vulcanization.
The tire components may include a belt member having a relatively low expansion rate. A tire having a belt member of relatively low expansion rate such as a belt member having an embedded cord directed substantially in the circumferential direction, can be manufactured using a full-mold vulcanization-molding machine applied with the present method.
Expansion rate of the belt member may be 3% and less. The present manufacturing method can be applied to a tire with a belt member having an expansion rate of 3% and less.
A full-mold vulcanization-molding machine having an upper mold and a lower mold may be used as a mold for vulcanizing and molding the green tire. Since the carved grooves extending about in direction of the lug groove are carved on surface of the green tire previously, a full-mold vulcanization-molding machine having upper and lower half split molds can be used. The vulcanization-molding machine is simple in construction, its installation space is small and its equipment cost is low. Especially, in case of a tire for a large-sized construction vehicle, the installation space and the equipment cost can be reduced much more.
The lug groove ribs on the upper and lower molds may be fitted in the carved groove of the green tire when the green tire is charged in the full-mold vulcanization-molding machine. Since the lug groove ribs of the upper and lower molds are fitted in the carved groove of the green tire, the lug groove rib cuts into the tread rubber easily and volume of displaced rubber becomes small. Therefore, pressing force by the lug groove rib is small, the belt wave and unevenness of the belt gauge caused by the pressing force acting on the belt member inside of the tread rubber can be restrained small as possible, joining of the joint portion is ensured, and generation of wrinkles can be prevented.
According to another aspect of the invention, there is provided a method for manufacturing a pneumatic tire, comprising: a step of manufacturing a green tire in which ribbon-like or sheet-like extruded rubber is piled up on a ply and a belt member assembled on a drum to form a tread; a step of forming a carved groove on a tread surface of the green tire in direction of a lug groove; and a step of charging the green tire formed with the carved groove in a vulcanization-molding machine to carry out vulcanization-molding.
In this method, the carved groove is formed on the tread surface of the green tire in direction of a lug groove after the step of manufacturing the green tire, then the green tire is charged in the vulcanization-molding machine to carry out vulcanization-molding. Therefore, pressing force by the lug groove rib is small, the belt wave and unevenness of the belt gauge caused by the pressing force acting on the belt member inside of the tread rubber can be restrained small as possible, and joining of the joint portion is ensured.
According to a further aspect of the invention, there is provided an apparatus for manufacturing a pneumatic tire which is a full-mold vulcanization-molding machine having an upper and a lower mold each provided with lug groove ribs, and guide means provided on the molds for letting the lug groove ribs fit to carved grooves formed on a green tire when the green tire is charged in the full-mold vulcanization-molding machine.
In this apparatus, the lug groove ribs of the upper and lower molds can be fitted in the carved groove of the green tire accurately owing to the guide means, so that volume of rubber displaced by the lug groove ribs cutting into the tread rubber is small. Therefore, pressing force by the lug groove rib is small, the belt wave and unevenness of the belt gauge caused by the pressing force acting on the belt member inside of the tread rubber can be restrained small as possible, and joining of the joint portion is ensured.